The Marines are doing the same. Before Iraq they had 8 regiments in 3 Marine Expeditionary Forces plus a Marine Reserve division:
The U.S. Marine Corps is disbanding some unneeded units (headquarters, anti-aircraft, artillery, and an anti-terrorism battalion) and activating the rest of the 9th Marine Regiment. The 9th Marines were deactivated in the early 1990s, but the 1st Battalion of the 9th was activated last year, and over the next two years, the other two battalions of the 9th will be activated. The marines are also reactivating a combat engineer battalion and some armored reconnaissance companies. The marines need the infantry and engineers in Iraq and Afghanistan, while the deactivated units have not been needed as much.
The Marines don't have the numbers or force structure to do as much as the Army since the Marines rely on Army logistics and other support units for lengthy campaigns.
But with 9 regiments relatively soon, the Marines should be able to rotate two regiments through Iraq and support 2-3 Marine Expeditionary Units at sea at any one time. And I assume the Marines are limited in calling up an already tapped out Marine reserve division to active duty.
